1. Field of the Invention
The present invention relates to a novel bidentate coordinated trivalent Group IVB metal complex, and, more particularly, to a process for preparing syndiotactic vinyl aromatic polymers using the bidentate coordinated trivalent Group IVB metal complex as a catalyst.
2. Description of the Prior Art
Vinyl aromatic polymers, or vinyl aromatic compound-containing polymers in general, can be classified into three categories: atactic, isotactic, and syndiotactic. The vinyl aromatic polymers having a syndiotactic configuration have many advantageous properties than the atactic and isotactic counterparts. For example, syndiotactic polystyrenes exhibit the advantages of having high melting point (about 270.degree. C.), low density (about 1.04 kg/cm.sup.3), low dielectric constant (about 2.6), high vicat softening temperature (about 254.degree. C.), excellent chemical resistance and water (steam) resistance, and low moisture absorption. The above reported values are measured from a specific syndiotactic polystyrene under a specific set of conditions. These values, of course, are subject to change as a result of different polymer molecular weight and many other factors.
In recent years, a trivalent titanium-based catalyst containing cyclopentadienyl as a ligand has been used for preparing syndiotactic vinyl aromatic polymers. For example, WO 95/10551 discloses a trivalent titanium-based catalyst composition for preparing syndiotactic vinyl aromatic polymers. Such catalyst composition includes pentamethylcyclopentadienyl titanium (III) dimethoxide Cp*Ti(OMe).sub.2, in which Cp* represents pentamethylcyclopentadienyl and Me represents methyl!, a borate, and a hydrocarbylation agent. In addition, European Patent No. 0,655,467 discloses the use of another trivalent titanium-based catalyst for preparing vinyl aromatic polymers.
However, the above-mentioned catalysts suffer from a number of shortcomings in that the catalyst has undesirable catalytic activity, inferior stereoselectivity, and the polymer so obtained exhibits an undesirably broad molecular weight distribution.